Lithium all-solid-state battery

ABSTRACT

An all-solid-state lithium battery, thermo-electromechanical activation of Li 2 S in sulfide based solid state electrolyte with transition metal sulfides, and electromechanical evolution of a bulk-type all-solid-state iron sulfur cathode, are disclosed. An example all-solid-state lithium battery includes a cathode having a transition metal sulfide mixed with elemental sulfur to increase electrical conductivity. In one example method of in-situ electromechanically synthesis of Pyrite (FeS 2 ) from Sulfide (FeS) and elemental sulfur (S) precursors for operation of a solid-state lithium battery, FeS+S composite electrodes are cycled at moderately elevated temperatures.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/371,500 filed Jul. 10, 2014, which is a national stage entry under 35U.S.C. 371 of International Application No. PCT/US2013/020819 filed Jan.9, 2013, which claims the benefit of priority to U.S. ProvisionalApplication No. 61/585,098 filed Jan. 10, 2012 and to U.S. ProvisionalApplication No. 61/590,494 filed Jan. 25, 2012. Each of theaforementioned application is incorporated herein by reference.

GOVERNMENT RIGHTS

This invention was made with government support under grant numberFA8650-08-1-7839 awarded by the Air Force Research Laboratory. Thegovernment has certain rights in the invention.

BACKGROUND

Use of traditional rechargeable lithium-ion batteries in consumerproducts is a concern due to safety issues. Such safety issues include,but are not limited to, solvent leakage and flammability of commercialgrade liquid electrolytes. Traditional solid-state batteries (composedof glass ceramic rather than liquid electrolytes) do not pose suchsafety risks and offer high reliability for end-users. However,traditional solid-state batteries suffer from poor rate capability, lowionic conductivity, interfacial instability, and low loading of activematerials.

Molten salt solid-state batteries require high operating temperatures(e.g., 400° C. and higher), so research was abandoned in search of roomtemperature lithium-ion and lithium-polymer technologies. Iron disulfidehas been successfully commercialized in high energy density primarycells. Unfortunately, sulfide conversion chemistries are irreversible atambient to moderately elevated temperatures, making these unusable forrechargeable batteries.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a high-level depiction of an example lithium or lithium-ionall-solid-state battery structure.

FIG. 2a shows for an example battery a field emission scanning electronmicroscope (FESEM) micrograph of synthetic FeS₂ that confirms cubicstructure with 2-3 μm cubes.

FIG. 2b shows for an example battery X-ray diffraction of syntheticpyrite.

FIG. 3a shows as an example of FeS₂ cycled at ambient temperature (about30° C.) and moderately elevated temperature (60° C.) in a liquid coincell and in an all-solid-state configuration for solid-state at 30° C.

FIG. 3b shows as an example of FeS₂ cycled at ambient temperature (about30° C.) and moderately elevated temperature (60° C.) in a liquid coincell and in an all-solid-state configuration for solid-state at 60° C.

FIG. 3c shows as an example of FeS₂ cycled at ambient temperature (about30° C.) and moderately elevated temperature (60° C.) in a liquid coincell and in an all-solid-state configuration for a liquid coin cell at30° C.

FIG. 3d shows as an example of FeS₂ cycled at ambient temperature (about30° C.) and moderately elevated temperature (60° C.) in a liquid coincell and in an all-solid-state configuration for a liquid coin cell at60° C.

FIG. 3e shows as an example of FeS₂ cycled at ambient temperature (about30° C.) and moderately elevated temperature (60° C.) in a liquid coincell and in an all-solid-state configuration for capacity retentioncomparison of cells cycled at 30° C.

FIG. 3f shows as an example of FeS₂ cycled at ambient temperature (about30° C.) and moderately elevated temperature (60° C.) in a liquid coincell and in an all-solid-state configuration for capacity retentioncomparison of cells cycled at 60° C.

FIG. 4a shows an example DFT simulation, which is a so-called“ball-and-stick” representation of Li_(x)FeS₂ along a charging cycle.

FIG. 4b shows an example DFT simulation, which illustrates the averageFe—Fe distance (d_(Fe—Fe)) at each state in comparison with the Fe bulkvalue.

FIG. 5a shows Coulometric titration results for a solid-state cell.

FIG. 5b shows dQ/dV of a solid-state cell.

FIG. 5c shows deconvolution of the dQ/dV peaks.

FIG. 6a shows a bright field transmission electron microscopy (TEM)image of electrode material from the solid-state cell cycled at 60° C.recovered after the twentieth charge for TEM.

FIG. 6b shows a high resolution (HR-) TEM image of electrode materialfrom the solid-state cell cycled at 60° C. recovered after the twentiethcharge for TEM.

FIG. 7a is a plot showing actual data obtained for Example 2.

FIG. 7b is a plot showing actual data obtained for Example 2.

FIG. 8a is a plot showing actual data obtained for Example 2.

FIG. 8b is a plot showing actual data obtained for Example 2.

FIG. 9a is a plot showing actual data obtained for Example 2.

FIG. 9b is a plot showing actual data obtained for Example 2.

FIG. 10 is a plot illustrating that activation of Li₂S exhibits ionicand electronic conductivity, as well as added thermal energy.

FIG. 11 is a plot showing cells cycled at C/5 and C/5 charge anddischarge rates.

FIG. 12a is a plot of an example rate study.

FIG. 12b is a plot of an example rate study.

FIG. 12c is a plot of an example rate study.

FIG. 12d is a plot of an example rate study.

FIG. 13a is a plot showing cycling data for a lithium metal cell.

FIG. 13b is a plot showing cycling data for a lithium metal cell.

FIG. 14 is an x-ray diffraction (XRD) spectra.

FIG. 15a shows x-ray diffraction (XRD) spectra.

FIG. 15b shows a TEM image.

FIG. 16a shows cyclic stability of a FeS+S/Li battery.

FIG. 16b shows a voltage profile of the FIG. 16a battery.

FIG. 16c shows a voltage profile of the FIG. 16a battery.

FIG. 16d shows a voltage profile of the FIG. 16a battery.

FIG. 17a shows dQ/dV profiles for bulk-type all-solid-state Li metalbatteries.

FIG. 17b shows a dQ/dV profile of an example battery.

FIG. 17c shows a dQ/dV profile of an example battery.

FIG. 17d shows a dQ/dV profile of an example battery.

FIG. 17e shows a dQ/dV profile of an example battery.

FIG. 18a shows XRD of FeS before and after milling.

FIG. 18b shows an FESEM micrograph of FeS.

FIG. 18c shows an FESEM micrograph of Fe_(1-X)S after mechanicalmilling.

FIG. 19a shows voltage profiles for two initially overcharged FeS/LiInbatteries.

FIG. 19b shows XRD measurements normalized with the (100) reflection ofthe beta-Be sample window.

FIG. 20 shows voltage profiles of a FeS/LiIn battery.

FIG. 21a shows specific discharge capacity of a battery as a function ofapplied current.

FIG. 21b shows voltage profiles of the same battery as a function ofapplied current.

DETAILED DESCRIPTION

Many advanced battery technologies are vying to be the successor oftoday's conventional Li-ion batteries. A strong argument can be madethat bulk-type all-solid-state lithium batteries (ASSLB) hold acompetitive edge in this technological race because they are inherentlysafe, have excellent shelf life, perform stably at high temperatures,and enable the reversibility of high capacity conversion batterymaterials like FeS₂. However, the energy density of high power ASSLBsmust be improved. The success of the ASSLB architecture can be realizedwith energy dense all-solid-state composite cathodes.

Examples of an ambient temperature, reversible solid-state cathode aredisclosed. An example implementation is in a lithium (Li) metalconfiguration. The battery may be constructed using a sulfideglass-ceramic solid electrolyte, and is implemented in anall-solid-state cell architecture. In an example, the battery may becharacterized as M_(x)S_(y)+zS where M═Fe, Co, Mo, y=0, 1, 2, 3 and z=0,½, 1, and so forth. This nomenclature is intended to include at leastthe following systems: FeS₂, FeS, FeS+S, and may also include othersuitable substitutes as will be understood by those having ordinaryskill in the art after becoming familiar with the teachings herein. Itis noted that the electrochemical synthesis of metal nano-particlesmaintains the electrochemical activity of Li₂S. Accordingly, the batteryaddresses issues previously associated with rapid capacity fade atambient temperature. The electrochemically driven synthesis oforthorhombic-FeS₂ (marcasite) can be at least partially achieved atambient temperatures.

The design of an ambient temperature transition metal plus sulfidebatteries is based at least in part on management of electro-activespecies formed upon full charge (2.5V versus Li+/Li) and full discharge(1.0V versus Li⁺/Li). Two example species are elemental iron)(Fe⁰ andpolysulfides (S_(n) ²⁻). To reduce or altogether prevent diffusion andagglomeration of Fe⁰ nanoparticles in conventional cells, a variety ofpolymer electrolytes have been employed with limited success.

A similar approach may be applied to the confinement of intermediatepolysulfides in conventional Li—S batteries. Example methods foraddressing intermediate polysulfide dissolution and Li₂S irreversibilityinclude polysulfide adsorption on high surface area CMK-3 nano-porouscarbon electrodes, polymer electrolytes, and polyacrylonitrile-surfurcomposites.

Another approach is to limit the upper and/or lower voltage bounds ofthe FeS₂ cells, for example, to about 2.2V and 1.3V respectively. Theformation of Fe⁰ and S_(n) ²⁻ is inhibited by avoiding full dischargeand charge. However, limiting the cell voltage range diminishesachievable energy density and subjects the cells to the risk ofover-charge or over-discharge.

The basic nature of an all-solid-state cell architecture allows for theconfinement of electro-active species. For example, FeS₂ and Li₂FeS₂ canboth be utilized reversibly as an all-solid-state anode. Anall-solid-state architecture reduces or altogether prevents Fe⁰dissolution and agglomeration. Sulfide-based, glass-ceramic solidelectrolytes and other materials that are stable at elevatedtemperatures demonstrate higher conductivities at ambient temperatures.Accordingly, lithium metal anodes can be safely used with solidelectrolytes because cell failure does not precipitate thermal runaway.A lithium metal electrode has a theoretical capacity of about 3876 mAhg⁻¹, is non-polarizable and has a low operating voltage that increasesachievable cell energy density. An all-solid-state architecture not onlyenables the safe use of a lithium metal anode, but also enables thereversible full utilization the cathode material.

Further examples herein disclose in-situ electrochemical formation ofhigh capacity conversion battery materials like FeS₂ and reversibleutilization of a glass or other stable electrolyte for higher overallelectrode energy density. In an example, the best performing compositeelectrode compositions are composed of no more than about 25% S or Li₂Sby weight. Sulfur's high theoretical specific capacity of about 1672 mAhg⁻¹ offsets poor active material mass loading so that high overallelectrode energy densities can be achieved for ASSLBs. To increase theoverall energy density of a composite electrode without changing thecomposition, the techniques described herein reversiblyelectrochemically utilize the glass-ceramic electrolyte. For example, byincorporating μm-Cu powder acetylene black, and 80Li₂S:20P₂S₅glass-ceramic electrolyte into a composite electrode, the Li₂S componentof glass-ceramic electrolyte electrochemical can be utilized. For moreeffective electrolyte activation and better electrode reversibility, theactive metal can be provided by in-situ electrochemical reduction.

The examples described herein may be further optimized by utilizing amechanochemically prepared active material nano-composite of highcapacity conversion battery materials like FeS and S. This materialprovides an alternative to the expensive solvothermally synthesizedcubic-FeS₂ (pyrite) based cathode. The precursors (e.g., FeS and S) arecomparatively inexpensive and can be obtained in much higher puritiesthan natural pyrite. The mechanical milting process also providesmaterial much more readily than the solvothermal method.

During testing, the rapidly increasing specific capacity of thenano-composite electrode (e.g., FeS+S) quickly exceeded its theoreticalcapacity by about 94% in testing. The excess capacity is a result of adramatic utilization of the glass electrolyte in the composite electrodewithout a degradation of cell performance. At its maximum, an exampleelectrode exhibited an energy density of about 1040 Wh kg⁻¹ which is thehighest energy density achieved for a bulk-type all-solid-stateelectrode. With extended cycling, the electrochemistry of the compositeelectrode (e.g., FeS+S) evolves a redox chemistry based primarily onthat of only sulfur. The results show that electrochemically structuredinterfaces between conversion active materials and the glass electrolytecan be utilized to increase energy density of ASSLBs, while maintaininggood rate performance.

It is noted that examples are described herein with respect to specificmaterials and process parameters for purposes of illustration only, andare not intended to be limiting. Other examples will be understood bythose having ordinary skill in the art after becoming familiar with theteachings herein, and are also intended to be included within the scopeof the claims.

Before continuing, it is noted that as used herein, the terms “includes”and including” mean, but is not limited to, Includes” or “including” andIncludes at least” or “including at least.” The term “based on” means“based on” and “based at least in part on.”

Lithium All-Solid-State Battery

FIG. 1 shows a high-level depiction of a lithium all-solid-state batterystructure 100. The battery structure 100 is shown including a cathode101, solid state electrolyte 102, and an anode 103. The arrows areillustrative of charge and recharge cycles. The anode may include alithium metal, graphite, silicon, and/or other active materials thattransfer electrons during charge/discharge cycles. In an example, thelithium reservoir may be a stabilized Li metal powder.

The cathode 101 is shown in more detail in the exploded view 101′,wherein the white circles labeled 110 represent a transition metalsulfide (e.g., FeS₂ or an mixture of FeS plus S). The gray circleslabeled 111 represent solid state electrolyte (SSE) particles. The SSEparticles promote ionic conductive pathways in and out of the cathode101. The black circles labeled 112 represent a conducting additive, suchas acetylene black.

In example, the transition metal sulfide (e.g., FeS plus S) may bemechanically mixed (e.g., using ball milling or other mechanicalprocesses), and are not chemically combined. The chemistry of thecathode approximates FeS₂ on the first cycle. After 10 or more chargecycles, the cathode exhibits a behavior that is similar to that of thefirst cycle.

Although the lithium all-solid-state battery structure 100 may beimplemented using any suitable transition metal sulfide plus sulfidecombination, the following discusses a specific battery structure basedupon synthetic iron disulfide.

Synthetically prepared FeS₂ is characterized with Field emissionscanning electron microscopy (FESEM) and x-ray analysis. FIG. 2 shows(a) a FESEM micrograph of synthetic FeS₂ that confirms cubic structurewith wide 2-3 μm cubes, and (b) X-ray diffraction of synthetic pyrite.The FESEM micrograph shown in FIG. 2(a) reveals cubic FeS₂ particleswith 3 μm wide faces. The X-ray analysis of synthetically prepared FeS₂shown in FIG. 2(b) shows diffraction peaks that match well with indexedpeaks.

Synthetic FeS₂ was tested in both an all-solid-state and liquid cellconfiguration. To achieve full utilization of FeS₂ the cells are cycledto full discharge (1.0V) and full charge (3.0V). The results of cyclingat ambient and moderate temperatures are shown in FIG. 3. For purposesof illustration, FIG. 3 shows FeS₂ cycled at ambient temperature (about30° C.) and moderately elevated temperature (about 60° C.) in a liquidcoin cell and in an all-solid-state configuration for: (a) solid-stateat 30° C., (b) solid-state at 60° C., (c) liquid coin cell at 30° C.,(d) a liquid coin cell at 60° C., (e) capacity retention comparison ofcells cycled at 30° C., and (f) capacity retention comparison of cellscycled at 60 ° C. All cells except for the 30° C. solid-state cell werecycled at a current of 144 μA which corresponds to a rate of C/10 forcharge and discharge. The 30° C. solid-state cell was cycled at rate ofC/10 for the first cycle and C/20 (72 μA) for all subsequent cycles.

Both solid-state cells are observed to have good capacity retention anda high degree of FeS₂ utilization. The gradual increase in capacity withcycling is observed to be a result of better FeS₂ utilization. Thisconclusion is supported by differential capacity (dQ/dV) analysis. Bythe twentieth cycle, the cell tested at 30° C. exhibits a dischargecapacity of nearly 750 mAh while the cell tested at S0° C. exhibits atheoretical discharge capacity of about 894 mAh g⁻¹. It is likely thatthe temperature dependence of solid electrolyte's conductivitycontributes to the full F_(e)S₂ utilization at 60° C., but not at 30° C.At 60° C. the conductivity of the 77.5Li₂S-22.5P₂S₅ solid electrolyteincreases to 4.4×10⁻³ Ω⁻¹ cm⁻¹ from 9.17×10⁻⁴ Ω⁻¹ cm⁻¹ at 30° C. Inaddition, a higher operating temperature increases the Li⁺ diffusivityin pyrite particles. More efficient Li⁺ insertion into cubic-FeS₂ isalso likely to result in better FeS₂ utilization.

In liquid cells, the discharge capacity rapidly fades upon cycling. Bythe twentieth cycle, the liquid cell tested at 30° C. exhibits adischarge capacity of only 190 mAh g⁻¹ while the cell tested at 60 ° C.exhibits no discharge capacity. Decomposition processes are acceleratedat 60° C. leading to such a fast rate of capacity fade that negligiblecapacity is observed after the second cycle. On the other hand, we havejust shown that cycling a solid-state FeS₂ cell at 60° C. only improvesits performance. At 60° C., it is possible to achieve a reversible, fourelectron utilization of FeS₂. It is noted that many traction batterypacks are designed to operate at temperatures near about 60° C. Thesuperior performance of all-solid-state batteries described herein athigher temperatures may reduce the need for extensive thermal managementsystems.

Mossbauer spectroscopy and near-edge X-ray absorption spectroscopy(XANES) show that the products of FeS₂ reduction are elemental iron(Fe⁰) and Li₂S. The initial discharge of FeS₂ proceeds in two steps:

FeS₂+2Li⁺+2e⁻

Li₂FeS₂   (1)

Li₂FeS₂+2Li⁺+2e⁻

2Li₂S+Fe⁰   (2)

Each reaction can occur at one voltage or two, depending at least inpart on the kinetics of the system.

Discharge profiles were observed having one plateau when the cell iscycled at 30° C., as can be seen in FIGS. 3(a) and (c), and two plateausat 60° C., as can be seen in FIGS. 3(b) and (d). At lower temperaturesthe reactions are limited by the Sow diffusivity of Li⁺ into the micronsized FeS₂ particles. At 30° C., reactions corresponding to equations(1) and (2) shown above proceed simultaneously at 1.5V versus Li⁺/Li Atmoderately elevated temperatures, equations (1) and (2) can proceed at1.7 and 1.5V respectively due to the higher diffusivity of If into themicron sized FeS₂ particles. The shoulder at 1.3V in the ambienttemperature liquid cell, as can be seen in FIG. 3(c), may be attributedto an irreversible side reaction of FeS₂ intermediaries with the organicelectrolyte. It is noted that the initial discharge profile of the FeS₂cells is much different than subsequent discharge profiles. Thevariation in discharge profiles may be due to the improved reactionkinetics of charge products compared to that of cubic-μmFeS₂. Betterkinetics may be due to changes in FeS₂ particle morphology and theelectrochemical formation of a different phase of stoichiometric of FeS₂like orthorhombic-FeS₂ (marcasite).

Superior performance observed in the solid state is believed to be dueto the confinement of electro-active species. The confinement of Fe⁰ bysolid electrolyte partially explains the better performance. Fe⁰ takesthe form of super-paramagnetic atoms or small aggregates of atoms ofabout 3.6 nm in diameter. Nano-particles of Fe⁰ have a high reactivitywhich is related to the nano-particle's large surface area. Should Fe⁰particles agglomerate into larger particles with smaller overall surfacearea, then these particles will have a lower reactivity. Without meaningto be limited by the theory, it may be the high reactivity of the Fe⁰nano-particles that maintains the electro-activity of Li₂S. Of course,other theories are also possible.

But Fe⁰ is susceptible to continuous agglomeration upon cycling.Agglomeration of Fe⁰ results in the isolation of Li₂S species and theobserved capacity fade when cells are discharged to low voltages. Anall-solid-state architecture can reduce or altogether prevent theagglomeration of Fe⁰ nana-particles. The atomic proximity of Fe⁰nanoparticles with Li₂S maintains the electro-activity of Li₂S withoutthe excessive amount of conductive additive needed in S/Li₂S basedbatteries.

An all-solid-state architecture is also successful at confiningpolysulfides S_(n) ²⁻ formed when the electro-active species present atfull charge are reduced. At ambient to moderate temperatures, FeS₂ isnot regenerated by the four electron oxidation of Fe⁰ and Li₂S. But thesame is not true for molten salt FeS₂ cells, which operate reversibly attemperatures in excess of 400° C.

Generally, subsequent charge and discharge cycles may proceed accordingto the following reactions:

Fe⁰+Li₂S

Li₂FeS₂+2Li⁺+2e⁻  (3)

Li₂FeS₂

Li_(2-x)FeS₂+xLi⁺+xe⁻ (05<x<0.8)   (4)

Li_(2-x)FeS₂

FeS_(y)+(2-y)S+(2-x) Li⁺+(2-x)e⁻  (5)

However equation (5) may be better represented by equation (6) based onthe results to be outlined below:

Li_(2-x)FeS₂

0.8ortho-FeS₂+02FeS_(8/7)+0.175S+(2-x) Li⁺+(2-x)e⁻  (6)

The direct reduction of sulfur by Li⁺ upon subsequent dischargetherefore introduces intermediate polysulfides (S_(n) ²⁻) into thesystem. In a liquid cell, polysulfides dissolve into the electrolyte andparticipate in a parasitic “shuttle” mechanism which causes rapidcapacity fade and self-discharge. The “shuttle” mechanism is the primarydegradation process occurring in sulfur-based cells. Polysulfides cannotdissolve into the solid electrolyte. Therefore, the confinement ofpolysulfides in an all-solid-state cell inhibits the “shuttle”mechanism.

Charge products at about 30-60° C. are likely a multi-phase mixture ofnano-particles of orthorhombic-FeS₂, non-stoichiometric FeS_(y) phaseslike pyrrhotite and elemental sulfur. In any case, the electrochemicallyactive products resulting from sequential charge cycles simulate theFeS₂ chemistry as well as provide electrical conductivity within theelectrode thus reducing the amount of conductive additive required. Thisconclusion is supported by the results of a DFT simulation shown inFIGS. 4-6.

FIG. 4 shows an example DFT simulation of lithiated Li_(x)FeS₂indicating material amorphization and Fe agglomeration for x=4, where(a) is a so-called “ball-and-stick” representation of Li_(x)FeS₂ along acharging cycle from x=4 to x=0, and (b) shows the average Fe—Fe distance(d_(Fe—Fe)) at each state in comparison with the Fe bulk value.

FIG. 5 shows: a) Coulometrci titration results for a solid-state celltitrated at 60° C. compared with the first second, and tenth dischargeprofiles for a solid-state cell cycled at 60° C., b) shows dQ/dV of asolid-state cell cycled at 30° C., and c) shows deconvolution of thedQ/dV peaks at 2.1 and 2.2V with fitted peaks and residual.

FIG. 6 shows electrode material from the solid-state cell cycled at 60°C. recovered after the twentieth charge for transmission electronmicroscope (TEM) analysis, where: (a) is a bright field TEM image of thetwentieth cycle sample with darker areas corresponding tonano-crystalline orthorhombic-FeS₂ and lighter areas corresponding to anamorphous region composed of FeS_(y) and elemental sulfur, and (b) is ahigh resolution (HR-) TEM of the twentieth cycle sample. FFT analysismatches with orthorhombic-FeS₂ along the [−110] zone axis.

Contrary to previous assumptions, subsequent discharges largely followthe same initial reaction path, instead, the difference between theinitial and subsequent discharge profiles is likely to changes inparticle morphology and the formation of the more open orthorhombic-FeS₂(marcasite). Thus, equation (6) more accurately describes the chemistryof subsequent cycles.

A study used coulometric titration to indicate that cubic-FeS₂ is notproduced electrochemically. However, the time needed for the FeS₂electrode to reach equilibrium is much longer than the 24 hours allowedin that study. When an FeS₂ cell is allowed up to about 144 hours toestablish equilibrium during initial discharge, the open circuit voltage(OCV) of the cell approaches the voltage of a subsequent discharge atthe appropriate reaction coordinate (x) as shown in FIG. 5(a). Thisresult indicates that the difference between the initial dischargeprofile and subsequent discharge profiles can be explained by kinetics.The difference is not a result of a different reaction pathway. Thediffusion of Li⁺ into the inner core of 2-3 μm pyrite cubes severelylimits the reaction rate of FeS₂ reduction, if electrochemicallyproduced FeS₂ particles are nano-crystalline, then the greatly increasedinterfacial surface area facilitates a fast reaction rate despite poorLi⁺ diffusivity. The diffusivity of Li⁺ may also be improved byregenerating a phase other than cubic-FeS₂. For example,orthorhombic-FeS₂ has a more open structure than cubic-FeS₂. Theformation of orthorhombic-FeS₂ instead of cubic-FeS₂ may result infaster Li⁺ diffusion, thus further increasing the reduction reactionkinetics.

High resolution transmission electron microscopy (HR-TEM) can be used tosupport this understanding through direct observation oforthorhombic-FeS₂ nano-particles upon charge. In an example, electrodematerial was recovered from the solid-state cell cycled at about 60° C.upon completion of the twentieth charge as can be seen in FIG. 3(b).This cell exhibits full utilization of FeS₂ so it is unlikely that asignificant mass of electrochemically inactive synthetic cubic-FeS₂remains in the cell by the twentieth charge.

FIG. 6(a) shows a bright field (BF) TEM image of the 20th cycled chargedFeS₂ solid-state electrode. This image depicts nano-crystalline domains(darker) of 100-200 nm in diameter encased by an amorphous material(lighter). Fast Fourier transform (FFT) analyses of HR-TEM imagesmatches well with orthorhombic-FeS₂ along the [-110] zone axis as can beseen in FIG. 6(b).

High resolution TEM imaging indicates a large amount of amorphousmaterial encasing the crystalline FeS₂ domains. To explore this issuefurther, the differential capacity of the all-solid-state cell cycled at30° C. was examined, as can be seen in FIGS. 5(b) and (c). The peaksshown in FIG. 5(b) correspond to the oxidation of Li₂S, and thereduction of S in an all-solid-state sulfur cell. The purple peakscorrespond to reaction plateaus observed in the solid-state FeS₂ cellscycled at 30° C.

When the solid-state FeS₂ cell is charged, no peaks are observedcorresponding to the oxidation of Li₂S. However, upon discharge a peakis observed at 2.2V, which corresponds to the direct reduction of sulfurto Li₂S. This result indicates that discharge and charge of a FeS₂all-solid-state cell at higher voltages at least somewhat followsequation (5), above. It indicates that sulfur is electrochemicallyproduced by the disproportionation of Li₂FeS₂. For this reason, theamorphous region is likely a mixture of elemental sulfur andnon-stoichiometric FeS_(y).

To quantify the amount of elemental sulfur produced upon charging, allelemental sulfur is said to be directly reduced to Li₂S at 2.2V. Thesolid-state cell cycled at about 30° C. exhibited a discharge capacityof about 737 mAh g⁻¹ upon the ninth discharge. If the peaks at 2.1 and2.2 V correspond to the reaction of charge products with two electrons,then integrating the dQ/dV curve between about 1.6 and 2.5 V yields acapacity 368 mAh g⁻¹ for this cell. When these two peaks arede-convoluted and fitted with a Voigt profile, the calculated total areagives a capacity of about 342.2 mAh g⁻¹, as can be seen in FIG. 5(c).This value matches well with the expected capacity of about 368 mAh g⁻¹.

The peak at about 2.2 V has an area of about 57.14 mAh g⁻¹, while thepeak at about 2.1V has an area of about 285.79 mAh g⁻¹. If (2-y)S isdirectly reduced to Li₂S, then the remaining capacity may be attributedto FeS_(y). The value of y can be determined to be about 0.085. Ifsubsequent discharges follow equation (5), then the chemical formula ofFeS_(y) is about FeS_(1.92). If FeS_(y) primarily takes the form ofFe₇S₈ (pyrrhotite), then the chemistry of subsequent cycles likelyfollows equation (6).

The charge products are likely a multiple phase mixture ofnano-crystalline orthorhombic-FeS₂, sulfur deficient phases of FeS_(y)and elemental sulfur. Accordingly, the charge products are believed tobe nano-crystalline orthorhombic-FeS₂ encased in amorphous sulfurdeficient FeS_(y) and sulfur (see FIG. 6(a)).

Coulometric titration indicates that the initial discharge iskinetically limited, and subsequent discharges follow a similar reactionpath (see FIG. 5(a)). Nano-crystalline orthorhombic-FeS₂ enables fasterreaction rates.

Sulfur reduction was observed at about 2.2V upon charge, but not Li₂Soxidation upon charge, as can be seen in FIG. 5(b). FeS_(y) phases andelemental sulfur may explain the amorphous domains observed in the highresolution TEM images. The presence of sulfur deficient FeS_(y) phasesand the observation of orthorhombic-FeS₂ is consistent,Orthorhombic-FeS₂ exhibits very weak temperature independentparamagnetism. For this reason, it is likely that ⁵⁷Fe Mossbauerspectroscopy used in previous studies was not capable of distinguishingorthorhombic-FeS₂ from other strong temperature dependent paramagneticphases like FeS_(y) and cubic- FeS₂. By way of example, Fe₇S₈ isferrimagnetic while FeS is paramagnetic. It is noted that ferrimagnetismis a much stronger effect.

The results of the DFT analysis shown in FIG. 4 indicate the formationof highly reactive atomic particles of Fe⁰. The fully-discharged,amorphous-like Li₄FeS₂ model (FIG. 4(a)) shows nanoscale separation of aFe⁰ nanocluster from Li₂S. The average Fe—Fe interatomic distance(d_(Fe—Fe)) at full discharge, x=4, is much shorter than that of Fe inthe bulk (FIG. 4(b)). A shorter d_(Fe—Fe) indicates that Fe⁰ should bevery catalytically active. Results also indicate the presence ofelemental sulfur as a charge product. At x=0, our atomic model depictssome degree of FeS₂ crystallization with a rather open structure (FIG.4a ). The x=0 model also depicts the presence of a S₂ dimer. It is thepresence of elemental sulfur that inhibits the full crystallization ofFeS₂ in our simulation. It is likely that the observed FeS_(2-y)nano-clusters could crystalize into orthorhombic-FeS₂ rather thancubio-FeS₂ because of the former's lower density.

Before continuing, it should be noted that the description of exampleambient temperature, reversible metallic lithium iron sulfide (FeS₂)solid-state batteries given above and further described below withreference to specific Examples is provided for purposes of illustration,and is not intended to be limiting. Other devices and/or deviceconfigurations using these and/or other materials may be utilized aswill be readily apparent to one having ordinary skill in the art afterbecoming familiar with the teachings herein.

EXAMPLE 1

In this example, the batteries discussed above were made for laboratoryscale analysis using commercially available polyvinylpyrrolidone (PVP,M_(w,avg)=10,000) and FeCl₂*4H₂O (>99%) obtained from Sigma Aldrich,ethylene glycol (99%) obtained from Mallinckrodt Baker Inc., and sulfurobtained from Fischer Scientific. HPLC grade water, analytical gradeNaQH, and absolute ethanol were used: without further purification. TheFeS₂ synthetic methodology used solvothermal reaction conditions.Dielectric heating for the reaction was provided with a microwavereactor. Microwave heating was selected because of its highreproducibility and the ability for automation, making this methodologyamenable to high throughput syntheses.

For the reaction 17 mL of ethylene glycol was added to 600 mg of PVP ina 35 ml microwave flask with a magnetic stirbar. Then 127 mg FeCl₂*4H₂O(0.64 mmol) was introduced. 8 mL of 1 M NaOH was then added, resultingin a dark green color. Finally, 180 mg of sulfur was added. Thissolution was stirred for 20 minutes while changing color from green toblack. Some sulfur remained undissolved during this process. Thereaction flask was then capped (70% full) and introduced to themicrowave.

The microwave used for this example was a Discover SP (CEM Inc.). Thesample was irradiated with 75 W of power until reaching 190° C., asmeasured by an infrared detector. The heating took about 7 minutes, andwas held at this temperature for 12 hours. Approximately 690 kPa ofautogenous pressure was generated. After the reaction was finished, theproduct was cooled by compressed air,

The resulting silver colored precipitate was separated by centrifugationand washed three times by sonication in ethanol. The precipitate wasthen stored in ethanol and vacuum dried overnight at 50° C. for batteryutilization. Synthetic FeS₂ was characterized by Cu-Kα x-ray diffraction(XRD) measurement, FESEM microscopy (JEOL JSM-7401F), and Ramanspectroscopy (Jasco NRS-3100).

Cell fabrication and cell testing for this example was carried out underan inert argon gas environment. The all-solid-state cells used in thisstudy were based upon the 77.5Li₂S-22.5P₂ 5 ₅ binary solid-stateelectrolyte. The composite positive electrode had a 10:20:2 weight ratiomixture of synthetically prepared FeS₂.77.5Li₂S-22.5P₂ 5 ₅, and carbonblack (Timcal Super C65), respectively. The composite positive electrodewas mixed using an agate mortar and pestle. Stabilized lithium metalpowder (SLMP) was used as the negative electrode (FMC Lithium Corp.).The construction of solid-state cells utilized atitanium-potyaryletheretherketone (PEEK) test cell die. 200 mg of solidelectrolyte powder was pressed at 1 metric ton in the PEEK cell die. 5mg of composite positive electrode and the stabilized lithium metalpowder were then attached to opposite sides of the solid electrolytelayer by pressing at 5 metric tons.

Liquid cells were fabricated by spreading an electrode slurry with a6:2:2 weight ratio of synthetic FeS₂, polyvinylfluorine (PVDF) binder(Alfa Aesar) and acetylene black (Alfa-Aesar, 50% compressed)respectively. PVDF binder was first dissolved intoN-methyl-2-Pyrrolidone (NMP) (Alfa-Aesar) solvent. FeS₂ and acetyleneblack were then stirred into the PVDF binder. A 50 μm thick layer ofslurry was spread on onto aluminum foil (ESPI Metals, 0.001″ thick) anddried at 60° C. in a single wall gravity convection oven (Blue M) for 5hours. To ensure good electronic contact, the electrode sheet was thencalendared with a Durston roiling mill to 75% of the total thickness.9/16″ diameter electrodes were punched and heat treated at 200° C. in anArgon environment overnight. FeS₂ electrodes were then assembled intocoin cells with a lithium foil negative electrode (Alfa-Aesar, 0.25 mmthick) and 1 M LiPF₄ electrolyte,

Cells were cycled galvanostatically using an Arbin BT2100 battery testerat room temperature (30° C.) and elevated temperature (60° C.). DeclaredC-rates were based upon FeS₂'s theoretical capacity of 894 mAh g⁻¹.Reaction equilibrium was studied by use of the galvanostaticintermittent titration technique (GITT).

EXAMPLE 2

FIGS. 7-9 are plots showing actual data obtained for Example 2.Specifically, FIG. 7 shows charge/discharge profiles for FeS₂ in a solidstate lithium battery, made from microwave synthesis. FIG. 8 showscharge/discharge profiles for in-situ formation of FeS₂ upon cycling ofFeS+S combined by planetary ball milling (cycled at C/10 ). The top plotin FIG. 9 shows the performance of FeS, with significantly loweredcapacity and a lack of voltage plateaus correlating to Eq. 7 and 8below. Upon discharge at room temperature, a slightly lower voltage isobserved due to internal resistance in the cell, and similarly forcharging with a slightly higher voltage. A significant decrease inreversibility is also observed, with first cycle coulombic efficiency ofboth room temperature and elevated temperature FeS cells below 75%. Thebottom plot in FIG. 8 shows a first cycle of FeS at (a) room temperatureand (b) elevated temperature.

In this example, solid state batteries utilizing highly conductingsulfide based solid electrolytes were used for reversible cycling ofFeS₂ electrodes in both room temperature (25° C.) and elevatedtemperature environments (60° C.) as shown in FIG. 7. Furthermore, itwas shown that there is no capacity loss for increasing rates up to C/8for elevated temperature testing. The first cycle shows the reactionformation of Li₂S and Fe down to 1.0V. Reversible cycling is allowed bythe highly reactive iron allowing successful de-Lithiation of Li₂S, andsubsequent formation of various Li—Fe—S compounds according to thefollowing equations which are visible in the charging profile.

3Li+2FeS₂Li₃Fe₂S₄(2.1V)   (7)

Li₃Fe₂S₄→2Li₂FeS₂(1.9V)   (8)

Li₂FeS₂+2Li→Fe+2Li₂S (1.6V)   (9)

After the first cycle, well defined voltage plateaus exist, showing thesuccessful formation of FeS₂ (from discharging) upon charging, andsubsequent formation of specific reversible Li—Fe—S phases upondischarge.

FeS₂ was formed in-situ during the first cycle (and also occurs over thecourse of many cycles), utilizing stoichiometric combinations of othermaterials. FeS and S were mixed either by mortar and pestle grinding, orby ball milting to produce an active material that is simply theaddition of both, without formation of FeS₂.

Upon the first discharge, plateaus corresponding to FeS and S werepresent, and resulted in a similar capacity of FeS₂ made by microwavesynthesis. By comparison (FIG. 8), it can be seen that there is adifference between discharge profiles for FeS₂ and FeS+S electrodes.However, these both resulted in the same highly reactive iron speciesthat allows the reversible cycling of Li₂S. Voltage plateauscorresponding to the similar peaks of FeS₂ occurred for FeS+S electrodes(FIG. 8) during charging. Subsequent discharging (2nd cycle) shows thatthese exhibited high reversibility and reaction plateaus for FeS₂,resulting from the FeS/S electrodes. Based on this understanding, it isfurther noted that another potential system includes electrochemicalformation of FeS₂ from elemental iron and sulfur being ball milted ormixed by hand.

Thermoelectrochemical Activation of Solid State Electrolyte

Thermoelectrochemical activation of solid state electrolyte is alsodisclosed herein. To increase cell energy density, it may be desirableto thermally activate the solid state electrolyte (e.g., Li₂S) insulfide-based solid electrolytes, including but not limited toxLi₂S-(100-x)P₂S₅. Initially charging a cell at an elevated temperatureincreases the energy density of the cell in one example by over 50%.

For purposes of illustration, two different composite electrodes werestudied: an 80Li₂S-20P₂S₅:acetylene black composite with a 20:1 weightratio respectively and a TiS₂:80Li₂S-20P₂S₅:acetylene black compositewith 10.20:1 weight ratio respectively. The cells in this example havean In metal negative electrode. However, the claims are not limited tothese electrodes, as suitable substitutes may be used as will beunderstood by those having ordinary skill in the art after becomingfamiliar with the teachings herein.

FIG. 10 is a plot illustrating that the activation of Li₂S is based atleast in part on both the sonic and electronic conductivity of TiS₂, aswell as added thermal energy (e.g., about 60° C.). The specific chargecapacities shown in FIG. 10 are based on the total mass of TiS₂ solidelectrolyte and acetylene black. Even at elevated temperature thecomposite with only solid electrolyte shows no capacity. Likewise, thecomposite with TiS₂ also shows no capacity when charged at roomtemperature.

However, the composite electrode with TiS₂ exhibited a charge capacityof 13 mAh g⁻¹ when charged at an elevated temperature of about 60° C.This corresponds to a specific charge capacity of about 40 mAh g⁻¹ basedupon the TiS₂ mass. As these cells have a lithium-ion configuration, andTiS₂ is already in the charged state, the only source of lithium inthese cells was Li₂S. Under an applied current at elevated temperature,it is believed that otherwise inert Li₂S is activated by the highlyionic and electronic conductive character of TiS₂. Other transitionmetal sulfides have similar material properties as TiS₂, and may beuseful in a similar Li₂S activation process.

FIG. 11 shows results of thermo-electrochemical activation of Li₂S usingnano-LiTiS₂. FIG. 11 is a plot showing cells with a 10:20:1 wt %nano-LiTiS₂:80Li₂S-20P₂S₅:acetylene black composite electrode cycled atC/5 and C/5 charge and discharge rates. The bottom data series (squares)shows the cell cycled at 30° C. (without thermal-electrochemicalactivation), and the top data series (triangles) shows the cellinitially charged at 60° C. before being removed to room temperature.The nano-LiTiS₂ was synthesized using mechano-chemical milling processinvolving Li₃N decomposition. The composite electrodes are a 10:20:1weight ratio mixture of nano-LiTiS₂:80Li₂S-20P₂S₅:acetylene black.

The cells in this example have an In metal negative electrode and arecycled at a rate of C/5 for both charge and discharge. However, theinitial charge and discharge cycles are both conducted at a rate ofC/10. Specific charge capacities presented are based upon the mass ofLiTiS₂ initially present in the composite electrode.

The first cell cycled at room temperature exhibits a very stablecapacity of about 230 mAh g⁻¹ after about forty cycles. The second cellundergoes an initial elevated temperature activation charge at about 60°C. It is then moved to room temperature (about 30° C.) for the firstdischarge and all cycles thereafter. This cell exhibits a 345 mAh g⁻¹discharge capacity after about the fortieth cycle. This represents abouta 119 mAh g⁻¹ (or about a 53%) increase in capacity over the theoreticalcapacity of LiTiS₂ of 226 mAh g⁻¹.

The increase in capacity observed with nano-LiTiS₂ is much larger thanthe 40 mAh g⁻¹ excess capacity achieved with theTiS₂-80Li₂S:P₂S₅-acetylene black composite. The greater surface area ofthe nano-LiTiS₂ particles is thought to more easily facilitate theactivation of Li₂S in the solid electrolyte. Supporting the previousfinding, the rate performance of nano-LiTiS₂ composite electrodes isshown in FIG. 12.

FIGS. 12(a)-(d) are plots of an illustrative rate study of a 10:20:1 wt% LiTiS₂: 80Li₂S-20P₂S₅:acetylene black composite electrode, where (a)and (b) are at 30° C., and (c) and (d) are at 60° C. The plots in FIGS.12(a)-(d) confirm the repeatability of results shown in FIG. 11. Thecell cycled at an elevated temperature exhibits discharge capacities inexcess of 160 mAhg⁻¹ greater than the theoretical capacity of 226 mAhg⁻¹for LiTiS₂.

The cells in this example have a Li metal negative electrode tofacilitate fast ion transfer. It is noted that the cell cycled atelevated temperature has a specific discharge capacity of nearly 390 mAhg⁻¹ at a rate of C/2 while the cell cycled room temperature onlyexhibits a capacity of 210 mAh g⁻¹ at C/2. Repeatedly charging atelevated temperature activates Li₂S in the solid electrolyte, providingexcess capacity.

FIGS. 13(a)-(b) are plots showing cycling data for a lithium metal cell.The plots show cycling data for a lithium metal cell with a 10:20:1 wt %FeS_(8/7)+S:77.5Li₂S-22.5P₂S₅:acetylene black composite cathode cycledat 60° C. At an elevated temperature, the cell gains capacity during thefirst 10 cycles. The increase in capacity is attributed to the S/Li₂Sredox reaction. The activation of excess sulfur is responsible for theevolution of a voltage plateau at about 2.2V. It can foe seen that thecell has a specific discharge capacity of about 807 mAh g⁻¹ on the firstdischarge, but a specific discharge capacity of 1341 mAh g⁻¹ by theninth cycle. This represents an activation of about 534 mAh g⁻¹. Theactivated capacity can be attributed to the evolution of the S/Li₂Sreduction plateau at about 2.2V.

As stated above, the particular solid electrolyte system in this exampleis xLi₂S-(100-x)P₂S₅. However, this technique is applicable to any Li₂Scontaining sulfide based electrolyte system is not limited toLi₂S—GeS₂P₂S₅ or Li₂S—SiS₂. These solid electrolytes are known as glassceramics. During electrolyte synthesis (e.g., by melt-quenching ormechano-chemical milling) Li₂S is incorporated into glass formers notlimited to GeS₂, P₂S₅, and SiS₂. Super-ionically conducting crystallinephases can also be precipitated in a glassy matrix upon subsequent heattreatment. It is also possible, that these crystalline phases maydecompose and result in some excess capacity.

FIG. 14 are x-ray diffraction (XRD) spectra of an example 80Li₂S-20P₂S₅solid electrolyte, example 10:20:1 wt % LiTiS₂:80Li₂S-20P₂S₅;acetyleneblack composite electrodes, and indexed spectra for Li_(1.0)TiS₂ andLi₂S. The XRD spectra of the solid electrolyte in this illustrationshows Li₂S peaking at about 27 and about 31.2 degrees, indicating thatthere are some Li₂S domains still present in the solid electrolyte. Itis believed that the additional capacity is a result of reacting excessLi₂S in the solid electrolyte. It is expected that the 77.5Li₂S-22.5P₂S₅solid electrolyte has less excess Li₂S to participate in activationreactions.

There is evidence of Cu_(y)S formation and Li₂S decomposition uponcharging at about 25° C. The cells in this example tend to exhibitinitial specific charge capacities of up to about 150 mAh g⁻¹.Composites without Cu tend to exhibit no capacity, indicating that Cu isa reacting species in the solid electrolyte.

The process described herein is analogous, but is also somewhatdifferent. That is, Cu reacts to form Cu_(y)S, while TiS₂ remainschemically/structurally stable. TiS₂ is an intercalation electrodematerial, while CuyS is a conversion battery material. TiS₂ succeeds inelectrochemically activating excess Li₂S because it is both highlyionically and electronically conductive. The process described herein isbased on an initial charging at elevated temperature, as no excesscapacity is observed at room temperature,

The iron sulfide (FeS₂, FeS_(x), or FeS_(x)+S) systems disclosed hereinare more like the Cu_(y)S electrodes. During lithiation (reduction),FeS₂ is not chemically/structurally stable like TiS₂. Instead, FeS₂reacts with 4Li⁺ in a conversion reaction to form the completely reducedproducts of Fe⁰ and 2Li₂S. Fe⁰ acts as a catalyst for the oxidation ofLi₂S. The products of oxidation include various electronicallyconducting phases of FeS_(x). These phases then help toelectrochemically activate excess Li₂S present in the solid electrolyte.

Electrochemical Evolution of a Bulk-Type All-Solid-State Iron SulfurCathode

The lithium all-solid-state battery described above, which may bethermally activated as described above, may also be made using a highcapacity conversion battery materials (e.g., FeS₂) equivalent. Examplesare described in the following discussing as in-situ electrochemicalformation of a FeS₂ phase and reversible utilization of a glasselectrolyte for higher overall electrode energy density. However, thelithium all-solid state battery is not limited to such animplementation.

The results described below show that electrochemically structuredinterfaces between conversion active materials and the glass electrolytecan be utilized to increase energy density of ASSLBs, while maintaininggood rate performance.

For purposes of illustration, synthesis of the iron sulfide basedall-solid-state composite electrodes can be by a three step planetaryball milling procedure (Across International, PQ-N2), An example77.5Li₂S-22.5P₂S₅ (molar ratio) glass electrolyte can be prepared bymilling about 0.832 g Li₂S (Aldrich, 99.999%, reagent grade) and about1.168 g P₂ 5 ₅ (Aldrich, 99%) in a 500 mL stainless steel vial (Acrossinternational) with two stainless steel balls (having about a 16 mmdiameter) and twenty stainless steel balls (having about a 10 mmdiameter) at about 400 rpm for about 20 hours.

The 1:1 molar ratio FeS:S active material composite (denoted as FeS+S)can be prepared by milling about 0.733 g FeS (Aldrich, technical grade)and about 0.267 g Sulfur (Aldrich, 99.98%) in a 100 mL agate jar (AcrossInternational) with five agate balls (having about a 10 mm diameter) andfifty agate balls (having about a 6 mm diameter) at about 400 rpm forabout 20 hours.

The composite electrode can be synthesized by milling a ratio ofprepared FeS+S, 77.5Li₂S-22.5P₂S₅, and carbon black conductive additive(Timcal, C65) in a 100 mL agate jar with five agate balls (having abouta 10 mm diameter) and fifty agate balls (having about a 6 mm diameter)at about 400 rpm for about 18 minutes.

In an example, cell fabrication and cell testing was carried out underan inert Argon gas environment, although other environments may also beutilized. The working electrode is about 5 mg of the mechanicallyprepared FeS+S based composite electrode, in this example, about 5 mg ofstabilized lithium metal powder (SLMP) was used as the counter electrode(FMC Lithium Corp., Lectro Max Powder 100). The shell of the solid statebattery was a titanium-polyaryletheretherketone (PEEK) test cell die. Tofabricate each cell, the glass electrolyte powder was first compressedat about 5 metric tons inside the PEEK cell die to form the separatorpellet. In this example, about 5 mg of composite positive electrode andthe SLMP were then attached to opposite sides of the glass electrolytepellet with about 5 metric tons force.

A variety of different batteries were fabricated to aid in thecharacterization of the FeS+S/Li battery's electrochemistry. Still otherexamples are contemplated, and the examples discussed herein are merelyillustrative. In an example, the FeS in these batteries was prepared bymechanically milling about 2 g of FeS in a 100 ml agate jar (AcrossInternational) with five agate balls (having about a 10 mm diameter) andfifty agate balls (having about a 6 mm diameter) at about 400 rpm forabout 20 hours. The cells used to electrochemically prepare the cycledXRD samples had a 165 mg FeS composite cathode and an InLi alloy anode.These cells operate at a lower potential because the InLi alloy has apotential of about 0.62V vs. Li⁺/Li.

In these examples, all cells were cycled under constant current constantvoltage (CCCV) conditions using an Arbin BT2000 battery tester at about60° C. Because the overall capacity of the FeS electrode is a movingtarget, rate performance is described by current and not by C-rate.Unless otherwise noted, specific capacities are given with respect tothe total mass of the composite electrode. Materials are characterizedby field emission scanning electron microscopy (FESEM, JEOL JSM-7401F)and Cu-Kα X-ray (XRD) measurement.

FIG. 15 shows (a) XRD of FeS+S composite active material and FeSprecursor with indexed reflections for Fe₇S₈ and FeS. The FeS precursormaterial is likely a multiphase mixture of FeS and an iron deficientFe_(1-x)S phase. After mechanochemical milling, only reflections for FeSand S are observed which indicates that no solid state reactionsoccurred to form FeS₂; and (b) is an FESEM micrograph of the FeS+Scomposite active material.

XRD measurement presented in FIG. 15(a) shows that the FeS precursor iscomposed partly of FeS (Triolite). However, the precursor also exhibitsferrimagnetism, which indicates that the sample is likely a multiphasemixture of FeS and an iron deficient phase like Fe₇S₈ (Pyrrhotite). Anunidentified peak at 44.64° suggests that other phases may be present aswell.

After mechanical milling with S, the reflections for FeS are observed todecrease in intensity and broaden. This is consistent with a decrease inaverage particle size by mechanical grinding action. Further, no newreflections are observed which suggests that the nano-composite is anintimate mixture of elemental FeS and S. The peak at 37.1° can beattributed to the strong (317) reflection of the S precursor (JCPDS#832285). FIG. 15(b) confirms that the FeS+S nano-composite is comprisedof sub-micron sized particles.

The electrochemical behavior of a FeS+S composite electrode over time iscomplex. FIG. 16 shows (a) cyclic stability of a FeS+S/Li battery.Assuming that the active material composition of FeS+S has a theoreticalspecific capacity of 900 mAh g⁻¹, the electrode's specific capacityshould not exceed 281 mAh g⁻¹. Excess capacity is evidence for theelectrochemical utilization of the 77.5Li₂S-22,5P₂S₅ glass electrolytecomponent; and (b)-(e) are voltage profiles for cycles 1, 2, 10, 20, 30,40, 60, and 150 of the same battery. The variability of voltage profileswith respect to cycle number suggests an evolving electrochemistry.

FIG. 16(a) shows the cyclic stability of a 5 mg electrode cycled at 60°C. with a current of 144 μA. The electrode exhibits an initial dischargeof 320 mAh g⁻¹ or 1020 mAh g⁻¹ based only on the mass fraction of theFeS+S nano-composite active material. The theoretical capacity of FeS+Sshould only be approximately 900 mAh g⁻¹ however, the other irondeficient phases present in the FeS precursor affect the actualtheoretical value. After the initial discharge, the electrode rapidlygains capacity until a maximum capacity of 550 mAh g⁻¹ and maximumenergy density of 1040 Wh kg⁻¹ are achieved by the 16^(th) cycle. Thismaximum is followed by an extended phase of capacity fade until theelectrode's capacity stabilizes at 330 mAh g⁻¹ by the sixty-eighthcycle. The initial discharge and the one-hundred-fiftieth cycle bothexhibit a specific capacity of very nearly 320 mAh g⁻¹ yet the energydensities of these cycles are 470 and 600 Wh kg⁻¹, respectively. Thediscrepancy in energy densities despite similar specific capacities is areflection of voltage profile evolution to higher potentials. FIGS.2(b-e) present the voltage profiles of the first and second, tenth andtwentieth, thirtieth and fortieth, and sixtieth and one-hundred-fiftiethcycles, respectively.

Voltage profile evolution can be correlated to the rise, fade andstabilization of the electrode's capacity. To understand the behavior ofthe FeS+S electrode, dQ/dV analysis was employed to qualitativelyidentify parallel redox chemistries. FIG. 17 shows (a) dQ/dV profilesfor bulk-type all-solid-state Li metal batteries with FeS, syn-FeS₂; andS composite cathodes; and (b)-(e) dQ/dV profiles for cycles 1, 2, 10,20, 30, 40, 60 and 150 of an example FeS+S/Li battery. It is observedthat the evolution of three parallel redox chemistries accounts for thechanging capacity of the battery. Twinning of reduction peaks in voltageranges between 2.13-2.19 and 1.56-1.51 V provides evidence for thein-situ electrochemical formation of a FeS₂ phase,

FIG. 17a shows the characteristic behavior of FeS, FeS₂, and S in anASSLB. The FeS₂ chemistry is characterized by dominant reduction peaksat 2.20, 2.14 and 1.49 V, while S is characterized by a reduction peakat 2.2V and FeS by a peak at 1.55V. The dQ/dV profiles in FIG. 17b forthe first and second cycles show evidence for the reduction of only FeSand S. As the cell's capacity increases, so too does the complexity ofthe voltage profiles. By the tenth and twentieth cycle, the dQ/dVprofiles in FIG. 17c show evidence for the reduction of FeS, S and FeS₂.The reduction of FeS₂ is apparent due to the twinning of reduction peaksin voltage ranges of 2.14-2.19 V and 1.51-1.56 V. This is the first timeevidence for the in-situ electrochemical formation of FeS₂ has beenpresented.

Next, capacity fade is correlated to the decline of peaks associatedwith Fe⁰←Fe²⁺ reduction and oxidation. The dQ/dV profiles in FIG. 17dfor the thirtieth and fortieth cycles during the capacity fade phaseshow that Fe redox peaks disappear while the S redox peaks remainrelatively stable. By the sixtieth cycle the dQ/dV profiles in FIG. 17efor the electrode achieve a degree of stability. Extended cyclingcapacity loss is associated with continued fade of the Fe redox peaksand the fade of S redox peaks.

The electrochemical utilization of the 77.5Li₂S:22.5P₂S₅ glasselectrolyte can be understood with reference to ex-situ XRD measurementof FeS based electrodes. In this example, FeS was used instead of FeS+Sto simplify the analysis. A glass electrolyte was used because theabsence of ceramic electrolyte diffraction patterns also simplifies theanalysis. To mimic the nano-size morphology of the FeS+S activematerial, the FeS used in this experiment was mechanically milled.

FIG. 18 shows (a) XRD of FeS before and after milling indicates that nophase changes occur during milling, (b) an FESEM micrograph of FeS asreceived from the vendor, and (c) an FESEM micrograph of Fe_(1-x)S aftermechanical milling. The XRD and FESEM analysis confirm that themechanical milting provides the needed size reduction. Utilization ofnano-FeS helps obtain good contact with the glass electrolyte.

FIG. 19(a) shows voltage profiles for two initially overcharged FeS/LiInbatteries. Composite electrodes were collected for XRD after fullovercharge (solid) and full discharge (dashed). As seen in FIG. 19(a),the FeS composite electrode was recovered after an initial overcharge(solid) and after full discharge (dashed) (FIG. 4a ).

In FIG. 19(b), all XRD measurements are normalized with the (100)reflection of the beta-Be sample window (dotted). Li₃S reflections areindicated by the dashed lines while Fe_(1-x)S reflections are indicatedby the solid lines. The XRD sample cycled to an initial full dischargeexhibits a low capacity because the electrode composite was mixedmanually and not mechanically. Because FeS is already in the fullycharged state, these cells should have exhibited zero charge capacity.Instead, both cells achieve an overcharge capacity of about 100 mAh g⁻¹.

It can be seen from the XRD of cycled FeS composite electrodes in FIG.19(b), that this capacity is associated with the utilization of excessLi₂S in the glass electrolyte component of the composite electrode. Thediffraction pattern for the glass electrolyte 1950 includes reflectionsfor Li₂S (dashed). As expected, the diffraction pattern for an uncycledcomposite electrode 1951 includes reflections for Li₂S and FeS (solid).After an initial overcharge 1952, the reflections for Li₂S disappear.When an electrode is discharged after an initial overcharge 1953,reflections for FeS disappear and weak reflections for Li₂S are onceagain detected. The reduced intensity of the Li₂S reflections in thissample is believed to be due to the small size of electrochemicallyprecipitated Li₂S particles.

To determine how reasonable it is to attribute the excess capacity tothe utilization of excess Li₂S in the glass electrolyte, the percentageof Li₂S oxidized in the cell presented in FIGS. 16 and 17 can beestimated by assuming that all of the FeS+S was electrochemicallyutilized and by acknowledging that the highest achieved capacity is 2.8mAh. A fraction of this capacity can be attributed to utilization of theglass electrolyte at the electrode/glass electrolyte separatorinterface, as indicated by FIG. 21.

FIG. 20 shows the first through seventeenth voltage profiles of aFeS/LiIn battery where the cathode is composed entirely of 5 mg ofnano-FeS. Normally, FeS should reversibly exhibit only a single plateau.However, with cycling, this battery develops a higher voltage plateau atapproximately 1.4 V versus. InLi. The evolution of the second plateaucan be attributed to the electrochemical utilization of Li₂S in theglass electrolyte at the electrode/glass electrolyte separatorinterface. By the seventeenth cycle, up to 300 μAh of capacity can beattributed to the utilization of glass electrolyte separator. Thisinterface utilization effect may lead to an overestimation of specificcapacity and contribute to the observed excess capacity.

Utilization of the glass electrolyte separator may lead to anoverestimation of specific capacity and contribute to the observedexcess capacity especially when the electrode is very small like it isin this case. The FeS+S component can account for 1.45 mAh of themaximum capacity. Without accounting for utilization of the glasselectrolyte separator, the remaining capacity indicates that 1.16 mg, or86%, of Li₂S in the glass electrolyte is electrochemically utilized,

Such a large percentage of the Li₂S component is likely not oxidizedwithout decreasing the ionic transport of the composite electrode. Yet,a rate test at 60° C. was conducted on another FeS+S electrode after afive cycle activation and good performance was observed.

FIG. 21 shows (a) specific discharge capacity of a battery as a functionof applied current, and (b) voltage profiles of the same battery as afunction of applied current. The electrode maintains a specific capacityof 200 mAh g⁻¹ with an applied current of about 2.40 mA. To maintaingood ionic transport, it is likely that the P₂S₅ component of the glasselectrolyte may be electrochemically utilized as well. Utilization ofP₂S₅ also explains why capacity increases during discharge cycles, andnot only during the charge cycles when excess Li₂S is initially oxidizedto S.

It is noted that while the initial specific discharge capacity of thecell presented in FIGS. 16 and 17 is similar to that presented in FIG.20, the degree and rate of capacity activation of each cell is muchdifferent. By the fifth cycle the first cell exhibits a specificcapacity of about 488 mAh g⁻¹ while the cell used in the rate test onlyexhibits a specific capacity of about 400 mAh g⁻¹. The maximum capacityof the rate cell is also about 15% lower than that achieved by the firstcell.

The electrodes for these two samples were prepared separately whichattests to the sensitivity of electrolyte activation to the quality ofthe glass electrolyte. Electrolyte sensitivity is emphasized by theresults of another study that did not observe electrolyte utilization.Like this study, nano-FeS was used as an active material and Li₂S was aprecursor for their electrolyte. However, the electrolyte was adifferent composition, thio-LISICON Li_(3.25)Ge_(0.25)P_(0.75)S₄, andwas prepared by melt quenching instead of mechanochemical milling.Previous work also did not show evidence for electrochemical activationof the glass electrolyte. In this study, three 5 micron cubes ofsynthetic FeS₂ were used as the active material. Such a large particlesize results in poor contact between the glass electrolyte and theactive material which may inhibit electrolyte utilization. Cubic-FeS₂ isalso a semiconductor with a much lower electronic conductivity than thatof the ferrimagnetic Fe_(1-x)S precursor used in this study.

It is noted that the decision to mechanically combine μm-Cu powder withS or Li₂S can be further improved. For example, the conversionmaterials, FeF₂ and CuF₂ suggest that a nano-structured network ofreduced metallic nanoparticles may be employed for good reversibility. Amechanical mixture with micron active metal particles is therefore notideal for good reversibility or for good electrolyte utilization.Instead, electrochemically reduced nano-active metal particles may beused due to the better atomic proximity to other reduced species, aswell as to the electrolyte particles, to further enhance reversibilityand effective electrolyte utilization.

The examples shown and described herein are provided for purposes ofillustration and are not intended to be limiting. Still other examplesare also contemplated.

What is claimed is:
 1. An all-solid-state lithium secondary battery,comprising: a cathode having a transition metal sulfide, wherein: uponfull discharge, the cathode undergoes conversion reactions to form atransition metal+lithium sulfide; and upon full charge, the cathodeundergoes conversion reactions to form the transition metalsulfide+lithium+electrons.
 2. The battery of claim 1, wherein thetransition metal sulfide is selected from monosulfides, disulfides, andtrisulfides.
 3. The battery of claim 1, wherein transition metal sulfideis mechanically combined.
 4. The battery of claim 1, wherein the cathodecomprises solid state electrode (SSE) particles, and a conductingadditive.
 5. The battery of claim 1, further comprising a solid stateelectrode (SSE) layer between the cathode and an anode.
 6. The batteryof claim 1, further comprising an anode including lithium metal,graphite, or silicon-based active materials.
 7. The battery of claim 1,wherein the cathode is selected from FeS₂ or FeS₂ equivalent.
 8. Amethod of in-situ electrochemical synthesis of pyrite (FeS₂) from ironsulfide (FeS) and elemental sulfur (S) precursors, comprising: cyclingFeS+S composite electrodes at moderately elevated temperature; whereincharge products are described by the following equation:Li_(2-x)FeS₂

0.8ortho-FeS₂+0.2FeS_(8/7)+0.175S+(2-x)Li⁺+(2-x)e⁻.
 9. The method ofclaim 8, further comprising producing voltage plateaus indicative ofFeS₂ in battery cells constructed with FeS+S or as an FeS₂ equivalent.10. The method of claim 9, wherein the voltage plateaus become moredefined upon further cycling.
 11. The method of claim 8, wherein themoderately elevated temperature is about 60° C.
 12. The method of claim8, wherein initial discharge of FeS₂ proceeds in two steps:FeS₂+2Li⁺+2e⁻

Li₂FeS₂   (1)Li₂FeS₂+2Li⁺+2e⁻

2Li₂S+Fe⁰   (2)
 13. The method of claim 12, wherein subsequent chargeand discharge cycles proceed according to the following reactions:Fe⁰+Li₂S

Li₂FeS₂+2Li⁺+2e⁻  (3)Li₂FeS₂

Li_(2-x)FeS₂+xLi⁺+xe⁻ (0.5<x<0.8)   (4)Li_(2-x)FeS₂

FeS_(y)+(2-y)S+(2-x) Li⁺+(2-x)e⁻  (5)
 14. A solid-state lithium battery,comprising: a solid state electrolyte; and an activating agent, whereinthe activating agent activates excess Li₂S in the solid stateelectrolyte to realize an improved charge capacity.
 15. The battery ofclaim 14, wherein solid state electrolyte is sulfide-based.
 16. Thebattery of claim 14, wherein activating agent is a transition metalsulfide such as FeS, TiS₂, FeS₂ and/or FeS₂ equivalent.
 17. The batteryof claim 14, wherein the activating agent has a highly ionic and/orelectrically conductive character.
 18. The battery of claim 17, whereinthe highly ionic and electrically conductive character of the activatingagent activates the solid state electrolyte.
 19. The battery of claim18, wherein the activating agent activates otherwise inert excess Li₂Sin the solid state electrolyte.
 20. The battery of claim 14, wherein theimproved charge capacity is realized after a single charge event at anelevated temperature.
 21. The battery of claim 20, wherein the elevatedtemperature is about 60° C.
 22. The battery of claim 20, wherein theimproved charge capacity is greater than about 50%.
 23. The battery ofclaim 14, wherein the sulfide based solid electrolyte isxLi₂S-(100-x)P₂S₅.
 24. The battery of claim 14, further comprising acomposite electrode.
 25. The battery of claim 24, wherein the compositeelectrode is 80Li₂S-20P₂S₅:acetylene black.
 26. The battery of claim 24,wherein the composite electrode is TiS₂:80Li₂S-20P₂S₅ acetylene black.27. The battery of claim 24, further comprising an In metal negativeelectrode.
 28. A method of activation of a solid-state lithium battery,comprising thermoelectrochemical activating excess Li₂S in a solid stateelectrolyte to realize an improved charge capacity.
 29. The method ofclaim 28, wherein the improved charge capacity is realized after asingle charge event at an elevated temperature.
 30. The method of claim29, wherein the elevated temperature is about 60° C.
 31. The method ofclaim 29, wherein the improved charge capacity is greater than about50%.
 32. The method of claim 29, further comprising an FeS₂ equivalentcathode.
 33. The battery of claim 1, wherein the cathode furtherincludes at least one of lithium sulfide and elemental sulfur prior tocharging and discharging.
 34. The battery of claim 1, wherein thelithium sulfide is at least one of a component of or mixed with theelectrolyte.